In a basic trunked system there exist a high degree of flexibility to partition voice conversations between different groups so that no one agency or district is aware that another agency or district is using the system. Typically, these groups are subdivided into subgroups and individuals so that call may be made using either a group, subgroup or individual ID code depending upon the type of communication desired by an initiating subscriber.
To establish a trunking voice communication between a group of units operating on a trunked system, it is known for a subscriber unit to send a data packet called a "inbound signalling word" (ISW). The ISW contains the requesting unit's unique ID code consisting of a group ID, an individual ID, and a subgroup code indicating which group of units are desired for communication. The request is forwarded to a central controller which decodes the request, and transmits a data packet called a "outbound signalling word" (OSW) to all subscriber units, which continuously monitor the control channel when not participating in a voice conversation. The OSW is a channel grant which contains the subgroup code of the requesting unit's ID code, and the voice channel number assigned for the conversation. The OSW causes the requesting unit to move to the voice channel and commence transmitting, while simultaneously causing all other subscriber units with a matching group ID to move to the voice channel as listening units. In this way a group call is set up. If, however, all voice channels are in use when a subscriber unit transmits an ISW, the central controller typically sends the requesting subscriber a "busy OSW".
In addition to group calls, it is often desirable for a subscriber to communicate privately with another subscriber. In prior trunked systems, individual or private communication was limited to a few preselected individuals. The call initiating subscriber selected one of the preselected few by a selection means such as a rotary switch or functional equivalent. Of course, a more desirable method of establishing a trunked private call may be envisioned by modelling the trunked communication system after the common telephone system. Thus, it would be desirable to merely enter a private ID number in a manner similar to dialing a telephone number to initiate a trunked private call. However, in practice, this improvement has been impossible to implement since the number of digital bits required to represent such a private ID number exceeds the maximum number of bits that the trunked system can communicate via the established signalling system. Accordingly, prior trunked radio communication systems suffer the detriment of having poor facilities to accommodate private subscriber calls.